Piezoelectric bimorph switches are known from the prior art. Such a switch may be regarded as an actuator. Actuators are regarded as a subdivision of transducers. They are devices, which transform an input signal (mainly an electrical signal) into motion. Electrical motors, pneumatic actuators, hydraulic pistons, relays, comb drive, piezoelectric actuators, thermal bimorphs, Digital Micromirror Devices and electroactive polymers are some examples of such actuators.
Microelectromechanical systems (MEMS) are the technology of the very small, and merge at the nano-scale into nanoelectromechanical systems (NEMS) and nanotechnology. MEMS are also referred to as micromachines (in Japan), or Micro Systems Technology—MST (in Europe). MEMS are separate and distinct from the hypothetical vision of Molecular nanotechnology or Molecular Electronics. MEMS are made up of components between 1 to 100 μm in size (i.e. 0.001 to 0.1 mm) and MEMS devices generally range in size from a 20 μm to a millimeter. They usually consist of a central unit that processes data, the microprocessor and several components that interact with the outside such as microsensors. At these size scales, the standard constructs of classical physics do not always hold true. Due to MEMS′ large surface area to volume ratio, surface effects such as electrostatics and wetting dominate volume effects such as inertia or thermal mass.
Various piezoelectric devices have been disclosed in the prior art.
U.S. Pat. No. 4,742,263 discloses a switch wherein piezoelectric bimorph elements are used to provide many separately controllable, closely spaced switchable contacts. The element includes at least two oppositely extending fingers connected by a common spine. The element spine is mounted to a case with the fingers spaced from an inner case surface. Electronic circuit means are mounted on the element spine for applying a separate electrical potential to each of the element fingers. A separate movable electrical contact is disposed on each of the element fingers spaced from the spine and insulated from the means for applying the separate electrical potentials to the fingers. A separate stationary contact is provided on an inner case surface opposite each of the movable contacts. In operations, a separate electrical potential is applied to each of the element fingers for selectively causing each finger to deflect and force its movable contact into electrical connection with the opposing stationary contact.
This disclosure relates to a bimorph piezoelectric switch with two applied voltages to drive the cantilever up- or downwards; it discloses very general claims, but it does not disclose a semiconductor device, or a MEMS. Further the piezoelectric layer is driven against the poling direction of more than a specific amount of the coercive field. Thus the switch is not actively opened to increase the opening gap, which is an issue especially in MEMS switches. Without any voltage the switch is not closed, but has a much smaller opening gap.
U.S. Pat. No. 4,595,855 discloses a synchronously operable electrical current switching apparatus includes a controllable piezoelectric relay having movable electrical contacts, and means for controlling the relay so as to move the contacts and effect a change of state of the electrical connectivity between them. The apparatus also includes means for operating the controlling mechanism so as to substantially reduce arcing between the contacts. A relay, which is especially useful in the apparatus, utilizes a piezoelectric bender and a flexible member disposed adjacent to one of the flat surfaces of the bender. The flexible member is fixed to the bender so that bending motion of the bender causes corresponding movement of the flexible member. At least one first electrical contact is disposed on the flexible member, and an electrical conductor having at least one second electrical contact is disposed adjacent the flexible member on the side thereof opposite the side which is adjacent to the bender. The electrical conductor is fixed to the flexible member so that movement of the member causes a change in electrical connectivity between the first and second contacts, and so that the electrical conductor is electrically isolated from the first contact when the first and second contacts are in an open position. The relay may be enclosed in a conventional 16-pin dual-in-line integrated circuit package, and is particularly useful for switching circuits operating at household power line current levels.
The switching apparatus, however, does not relate to a semiconductor device, let alone a MEMS. Further, the proposed solution suffers from e.g. residual stress gradient, single clamped beams, etc. In fact only a bimorph is mentioned; such a bimorph does not have a stress gradient free cantilever over the whole operation temperature range.
U.S. Pat. No. 4,625,137 discloses a piezoelectric actuator using a bimorph element, comprising: a bimorph element having at least one pair of piezo elements respectively on two surfaces of a central electrode and an electrode formed on a surface of each piezo element which is opposite to a surface contacting the central electrode, the piezo elements formed on the two surfaces of the central electrode being polarized in the same direction with respect to a direction of thickness of the bimorph element; a drive input circuit arranged such that a cathode of a first directional voltage limiting circuit is connected to one electrode contacting a positively polarized surface of the piezo element whose negatively polarized surface contacts the central electrode, that an anode of a second directional voltage limiting circuit is connected to the other electrode contacting a negatively polarized surface of the piezo element whose positively polarized surface contacts the central electrode, that an anode of the first directional voltage limiting circuit and a cathode of the second directional voltage limiting circuit are commonly connected to constitute one drive input terminal, and that the central electrode serves as the other drive input terminal; a constant voltage source for applying a constant voltage to the drive input circuit; and a polarity switching circuit, arranged between the constant voltage source and the drive input circuit, for switching a polarity of the constant voltage source with respect to the one and the other drive input terminals of the drive input circuit.
This disclosure relates to a piezoelectric actuator using a piezoelectric bimorph; it mentions the problem of depolarization in case of series and parallel-switched bimorphs; it circumvents this problem by polarizing again the piezoelectric crystal after every depolarization. The disclosure relates to the polarity switching voltage supply to prevent depolarization. It does not relate to a semiconductor device, nor to a MEMS, Further, the layout of the switch is not mentioned as such.
U.S. Pat. No. 4,553,061 discloses a direct current latching relay comprised of bender-type piezoelectric drive members each of which is a three terminal member formed of two piezoelectric plate elements separated by a conductive plane. Each piezoelectric plate element is separately electrically charged with an input pulsed DC switching signal of the same polarity as the pre-poling field previously induced in the piezoelectric plate element. By electrically charging one of the piezoelectric plate elements with a switching signal electric field of the same polarity as the pre-poling field, the bender-type drive members are member made to bend in one direction. Alternatively, by charging the opposite plate of the drive member again with a direct current electric charging field of the same polarity as the pre-poling field previously induced in the plate, the bender-type drive member can be caused to bend in the opposite direction. The bender-type piezoelectric drive member when bent engages and drives a push rod which actuates a snap-action switching contact mechanism from either an open circuit state or to a closed circuit state or vice versa. Pulsed direct current charging fields are applied to the piezoelectric plate element of the bender-type drive member and a high resistance discharge resistor is connected across each of the piezoelectric plate members so as to automatically discharge the plate members shortly after their excitation. As a result, no long-term depolarization of the piezoelectric plate members occurs and because of the pulsed short-term nature of the charging fields, no long-term deformation (creep) develops in the plate elements over extended periods of usage of the relay.
The above piezoelectric bimorph driven direct current latching relay actuates only in poling direction. However, it relates to a macroscopic relay using snap action to create a latching relay; thus, this is a macroscopic switch which does not suffer from residual stress gradients; compensation of the thermal expansion coefficients is not mentioned; in this patent the actuation forces are increased by stacking several bimorphs, which is very challenging in present thin film technique, if possible at all. As such it does not relate to a semiconductor device, or to a MEMS.
U.S. Pat. No. 7,098,577 B2 discloses a piezoelectric switch for tunable electronic components comprising piezoelectric layers, metal electrodes alternated with the layers and contact pads. Cross voltages are applied to the electrodes, in order to obtain an S-shaped deformation of the switch and allow contact between the contact pads. Additionally, a further electrode can be provided on a substrate where the switch is fabricated, to allow an additional electrostatic effect during movement of the piezoelectric layers to obtain contact between the contact pads. The overall dimensions of the switch are very small and the required actuation voltage is very low, when compared to existing switches.
The piezoelectric switch describes series and parallel arrangements of a bimorph switch (see for instance FIG. 1-3, 7, 8): structured center electrodes to achieve an S form bending are shown. The switch does not compensate for the various effects mentioned e.g. above.
US2002064001 discloses a piezoelectric device comprises an integral body of piezoelectric material having a length and width greater than its thickness; wherein the device further includes, in contact with the body, respective pairs of electrodes, wherein the electrodes of each pair are opposed in the thickness direction, wherein the body deflects along said transverse direction when a voltage is applied to the electrode pair. The invention further provides a disk drive suspension and a head gimbal assembly.
US2004135472 discloses a method, wherein a piezoelectric element has an asymmetrical hysteresis characteristic of polarization-electric field different in absolute values between a coercive field of positive electric side and a coercive field of negative electric field side, and is polarized in the film thickness direction of the piezoelectric element and in a direction of smaller absolute value of the coercive field, and as a position control voltage Q1 for position control by ditorting the piezoelectric element in a direction orthogonal to the film thickness direction of the piezoelectric element, a voltage equivalent to an electric field of 0.4 or less of the coercive field value is applied in the film thickness direction of the piezoelectric element in a direction of larger absolute value of the coercive field. Further, a position control voltage Q2 for recovering from deterioration of polarization of the piezoelectric element is applied by superposing on the position control voltage Q1, or changing over with the position control voltage Q1, or while the position control voltage Q1 is not applied, and therefore recovering from deterioration of polarization characteristic, the displacement characteristic is stabilized for a long period of time.
WO9427330 discloses a bimorph apparatus having an extended range of displacement is disclosed herein. The bimorph apparatus is typical in that it includes a bimorph having a conductive vane sandwiched between first and second piezoelectric layers, free for movement in response to electrical signals applied thereto. The bimorph includes a driver circuit for developing a first potential drop across the first layer, which induces contraction thereof. In addition, however, a second driver circuit operates to develop a second potential drop different from the first potential drop across the second layer contemporaneous with development of the first potential drop, thereby inducing expansion of the second layer. In this way simultaneous contraction and expansion of the first and second layers results in extended displacement of the free portion of the bimorph in a first direction. The second driver circuit may be adapted to apply a third electrical signal to the second layer, and the first driver circuit implemented to apply a fourth electrical signal to the first layer simultaneously with application of the third signal to the second layer. In this manner extended movement of the free portion of the bimorph is induced in the opposite direction or some other second direction.
GB2012106 discloses an electro-mechanical transducer having a dielectric plate which is coated on opposite major surfaces with electrodes, and a base plate which is bonded to one of the major surfaces of the dielectric plate through the electrode and does not follow the shrinkage and expansion of the dielectric plate when a voltage is applied to the dielectric plate through the electrodes perpendicular thereto. The dielectric plate is made of a ceramic, which is in paraelectric phase or anti-ferroelectric phase at room temperature to avoid residual strain and displacement due to domain structure. Two ceramic plates may be bonded with a phosphor bronze base plate there between to form a bi-morph. Such transducer can be used to support a magnetic head in a video tape recorder.
None of these documents, however, disclose a stack compensating for thermal expansion coefficients and other effects mentioned above. Further, WO9427330 and GB2012106 do not relate to MEMS.
A disadvantage of typical bimorph actuators is that electric field is not always applied in the same direction as the poling direction of the piezoelectric layers. As a consequence depoling of the piezoelectric layer is present and the lifetime of the switch jeopardized.
Another main property of a typical prior art bimorph switch suffers from a bending moment and a high sensitivity to ambient temperatures, specifically if residual stresses are present in the layers and the temperature expansion coefficient is different across the layer stack, respectively.
Normally, a cantilever is not actively opened, and so the initial gap can not be chosen to be very small. Because this would lead to a high capacitive coupling in the open state, low self-pull in voltage (i.e. electrostatic closing of the switch by a high RF power) and small restoring forces. Small restoring forces will increase the probability for sticking of the contacts in the closed state due to adhesion forces.
Thus, the prior art bimorph switches exhibit one or more of the above disadvantages.
As a consequence there still is a need for improved bimorph actuators.
The present invention intends to solve one or more of the above mentioned problem, by providing an improved bimorph switch. The present switch, on the other hand, will remain to exhibit other advantages of prior art switches.